The unevenness in specific gravity and strength of composition boards made by conventional processes becomes fairly conspicuous when the board is subjected to a secondary processing, such as painting, laminating, cutting, etc. This unevenness results in a lowering of the yield ratio of usable board material relative to total produced board material.
Studies by the present applicants reveal that the main cause of the unevenness of specific gravity and strength of the composition board does not depend merely on variations of the application rate of adhesive or binder, as generally thought, but rather that, as a phenomenon arising from the dry manufacturing process of the composition board, the density unevenness of fiber and/or grain comprising the mat constitutes another principle cause of uneven characteristics in the composition board. hence, particular attention has been paid to minimization of density unevenness of the fiber and/or grain in the mat, along with uniformity of application or spreading rate of the adhesive.
The ultimate key object of our invention is to procure a composition board whose strength distribution is uniform throughout, such that further processing or work on or with the board can be effected with high accuracy and efficiency.
Other objects and characteristics of the invention will be clear from the ensuing explanations and description.
Although the ensuing description deals with an embodiment of the invention utilizing wood fiber in making the dry fiber mat, it will be understood that various fibrous or granular materials as conventionally used are applicable to the instant invention, together with appropriate binders for such materials.
In conventional manufacturing processes for dry fiber mats, the fiber is conveyed in an air stream from a refiner and is sprayed and coated with a suitable adhesive or binder. After separation from the air stream by a cyclone separator, the fiber is formed into mats on a wire screen belt conveyor to a given height and size, this conventionally being done by forming or felter apparatus. However, the quantity of fiber conveyed per unit of time to the former, after adhesive is applied in a mixer, varies irregularly, increasing or decreasing. This irregular increase or decrease of the air-conveyed fiber quantity is attributed to the fact that the raw material chips that are treated by the refiner are controlled not by weight, but by volume, so the bulk density varies according to the shape, kind of wood, etc., and this makes the weight and bulk density of the fiber discharged from the refiner irregular in accordance with changes in the refining conditions.
When the fiber is formed into a mat on the wire screen conveyor by the former, there is hardly any problem with variations transversely across the mat, but in the longitudinal direction a noticeable unevenness of bulk density occurs.
In the conventional process, the formed mat is shaved to an equal or uniform thickness by shave-off rolls disposed immediately after the forming apparatus or immediately after individual formers, such that the mat appears uniform, but the bulk density of the fiber deposited from the former onto the wire screen conveyor running at a given speed varies not only in accordance with changes in the quantity of fiber conveyed to the former, but also in accordance with suction from the vacuum box or boxes underneath the wire-netting conveyor. Accordingly, where the fiber is deposited with relatively greater thickness, the bulk density of the mat rises due to the suction strength from the vacuum box or boxes. In either case, even if the mat thickness is thereafter uniformly shaved off by the shave-off roll or rolls, the problem of unevenness of bulk density remains unsettled.
In order to operate a rejector continuously measuring the weight and bulk density of formed mats so as to reject or remove those mats exceeding a predetermined range of density variations, it is known to use a gamma ray density measuring device, measuring spots or points on a finished mat, but such device has not solved the problem of inequality of bulk density and weight of mats occurring during the forming process.